The mechanism of corrosion has been determined to be electrochemical in nature. The corrosion of underground metal structure or even structure exposed to moisture in the atmosphere is the result of cathodic disbondment or ions flowing from the metal structure to the surrounding electrolyte. Corrosion is particularly hard on underground pipes or conduits, such as gas distribution systems. Corrosion is also particularly hard on fittings used in such systems. The extent of corrosion may vary widely throughout a distribution system and depends on many factors such as the chloride content or acidity of the soil or surrounding electrolyte.
Cathodic protection systems are employed to minimize corrosion. These systems are either galvanic or of the impressed current type. The former utilizes a natural current flow between metals of different types while the latter actually impresses a current through the electrolyte from an anode to the buried structure as a cathode. The anode then corrodes rather than the buried structure. One drawback of the latter system is that it must from time to time be tested. Testing takes place by measuring the potential from a reference cell to the structure with the system off.
It has been found that fittings for such systems won't corrode in typical fashion if one side is electrically insulated from the other. In other words, if the tube connected is electrically insulated from the fitting, the fitting is insulated from the cause of corrosion. An insulated fitting also makes cathodic protection for the system easier to apply, control and monitor.
While there are many plastic fittings or fittings using plastic parts, such as a typical garden hose connection, the really quality fittings more suitable for a gas distribution system are such as those used in process control piping. Such fittings are made of metal. Examples of such fittings are seen in prior U.S. Pat. Nos. 3,736,008 and 3,069,188 and are sold under the well known TYLOK.RTM. trademark by Tylok International Inc. of Euclid, Ohio. These fittings typically comprise a precision made body, a nut and two collets, the latter providing four axially spaced sealing points along the tube. The nut is spaced from the tube since the rear collet includes a sleeve telescoped between the nut and tube. The body includes different axially spaced cam or taper surfaces which actuate the collets. The fitting when properly installed is substantially stronger than the tube. Since the collets actually skive into the tube, the fitting is a good electrical conductor, and in some applications that electrical conductivity is desirable.
It would therefore be desirable to maintain the strength and integrity of the fitting using interchangeable parts, such as the nut and body, yet providing a fitting which electrically insulates the fitting from the tube.